Electricity is an indispensable energy nowadays. No matter lighting devices, home appliances, communication apparatuses, transportation, or industrial equipment. without electricity, none can operate. Current global energy mainly comes from burning petroleum or coal. However, the supply of petroleum or coal is not inexhaustible. If people don't search actively for alternative energy, when petroleum or coal is exhausted, the world will encounter energy crisis. For solving the problem of energy crisis, in addition to developing positively various kinds of renewable energy, it is required to save energy and use energy efficiently for improving the usage efficiency of energy.
Take lighting equipment as an example. Light equipment is indispensable in human lives. As technologies develop, lighting tools having better luminance and more power saving are gradually provided. Currently, an emerging light source is LED. In comparison with light sources according to prior art, LEDs have the advantage of small size, power saving, good light emitting efficiency, long lifetime, fast response time, no thermal radiation, and no pollution of poisonous materials such as mercury. Thereby, in recent years, the applications of LEDs are wide-spreading. In the past, the brightness of LEDs still cannot replace the light sources according to the prior art. As the technologies advance, high-luminance LEDs (high-power LEDs) are developed recently and sufficient to replace the light sources according to the prior art.
The epitaxial structure of LED is composed of semiconductor layers of p-type and n-type gallium-nitride family and light emitting layers. The light emitting efficiency of LED is determined by the quantum efficiency of the light emitting layer as well as the extraction efficiency of the LED. The method for increasing the quantum efficiency is mainly to improve the epitaxial quality and the structure of the light emitting layer; the key to increasing the extraction efficiency is to reduce the energy loss caused by reflection of the light emitted by the light emitting layer within the LED.
Depending on the property of the material of the p-type semiconductor layer and the work function of the metal used as the reflection layer, an Ohmic-contact or a Schottky contact is formed between the p-type semiconductor layer and the reflection layer of a general LED. When the resistance of an Ohmic-contact is too high, the operating characteristics of LED will be affected. It is thereby required to lower the resistance of the Ohmic-contact. The Ohmic-contact characteristics between the p-type semiconductor layer and the reflection layer can be improved by disposing an Ohmic-contact layer therebetween. The Ohmic-contact layer according to the prior art adopts a Ni/Au Ohmic-contact layer and heat treatment is performed on the Ohmic-contact layer for forming a good Ohmic-contact. Nonetheless, the light absorption rate of the Ni/Au Ohmic-contact layer is higher. Besides, the interface between the p-type semiconductor layer and the Ni—Au Ohmic-contact layer is roughened due to the heat treatment and leading to inability in reflecting light. Consequently, the reflection efficiency of the LED will be reduced.
For solving the problems described above, please refer to FIG. 1, which shows a structure diagram of the LED device according to the prior art. As shown in the figure, an Ohmic-contact layer 11′ is disposed between the p-type semiconductor layer 10′ and the reflection layer 12′. The Ohmic-contact layer 11′ uses a single-layer metal-oxide layer and has high electrical conductivity. Although the Ohmic-contact layer 11′ has high electrical conductivity, it lowers the light transmittance, and hence leading to lowering of the light emitting efficiency of the LED. If the Ohmic-contact layer 11′ has high light transmittance, its electrical conductivity will be reduced, making the Ohmic-contact characteristics of the Ohmic-contact layer 11′ inferior. Thereby, the Ohmic-contact layer 11′ according to the prior art, which adopts a single-layer metal-oxide layer, cannot have good Ohmic-contact characteristics while maintaining superior light emitting efficiency.
Accordingly, the present invention provides an LED device and a flip-chip packaged LED device having excellent Ohmic-contact characteristics as well as superior light emitting efficiency.